Parliament will this week be asked to make the most serious single assault on liberty in memory. The witness anonymity bill abolishes the right of defendants to know the identity of their accusers. This will result in thousands of unfair trials; and the principles of open justice, which this nation has contributed to the lexicon of human rights, will be gutted by a panic-stricken measure that encourages courts, in criminal cases of any kind, to suppress the identity of crucial witnesses.

The bill is being rushed through before the summer recess to empower all criminal courts and courts martial to receive voice-distorted evidence from witnesses whose identity defendants and their lawyers will never be allowed to know and whose faces they will never see. Defendants could be imprisoned for life solely on secret evidence they can never test by cross-examination so as to reveal, for example, a witness's malice or personal animosity; spiteful or score-settling motives; a reputation for telling lies or devious relationships with the police. Such witnesses will now be handed a perjurer's charter, by way of a statutory "anonymity order" that will keep their identity for ever hidden.

The bill will in effect place the trial process in the hands of the police, who will offer anonymity in most investigations into violent crime; the prosecution will be permitted to make an application in secret to the trial judge to claim that witnesses will not testify unless an anonymity order is made. The judge will have no way of weighing this claim, because the defence will not be present to challenge it. On many occasions at these one-sided hearings, judges will give in to the untestable claim (a form of forensic blackmail) that without these orders trials cannot proceed.

There are no safeguards for the citizen. The prosecution does not even have to prove that a witness has been intimidated or fears any kind of mental or physical threat: any "harm to the public interest" is sufficient - a formula that might cover up questionable police operations. There is no safeguard against a conviction relying entirely on the evidence of an anonymous witness; incredibly, this bill does not require judges to ensure corroboration (independent evidence pointing to guilt), or even to warn juries about the dangers of convicting on the word of witnesses who can't be effectively cross-examined. There is no right of appeal against the granting of anonymity orders.

The bill is prefaced by a statement from Jack Straw that it conforms with the European Convention on Human Rights. It does not: article six of the convention says that "everyone charged with a criminal offence" has, at minimum, a right "to examine or have examined witnesses against him" - and you cannot examine a distorted voice.

This provision, and the sixth amendment of the US constitution that enshrines the right of Americans to confront accusers, derives from English precedents that go back to the great achievement of the long parliament in 1641 in abolishing the court of star chamber, with its hated reliance upon anonymous witnesses and torture; and to "Freeborn John" Lilburne, the Leveller leader who insisted that all proceedings at his treason trial be heard in open court. For centuries thereafter, this nation could boast of the fairness of its trials compared with other countries in Europe where, as Jeremy Bentham stated, evidence was heard beneath a "veil of secrecy" and "wide open to mendacity, falsehood and partiality".

The witness anonymity bill is the result of a panic that followed last month's ruling by the law lords that, under the principles of English common law and the European Convention on Human Rights, a defendant could not be convicted "solely or to a decisive extent upon the testimony of one or more anonymous witnesses". This should never have been doubted: as a matter of common sense, no trial can be fair if critical evidence cannot be challenged. However, in 2006 the court of appeal erroneously endorsed this unlawful practice, which led to a flood of applications from the police - some 600 are said to have been granted. One unedifying reason for rushing this bill through is to validate, retrospectively, orders that were unlawful when they were made.

The police claim that they cannot secure convictions without anonymity orders because of an increase in intimidation. But this is not a new problem: the Krays and Richardsons terrorised London, yet were convicted without resort to secret witnesses. At the height of intimidation in Northern Ireland, Lord Gardiner (Jack Straw's greatest of predecessors) rejected a proposal for witness anonymity for the very reason that it infringed the right to a fair trial. In the US and Italy today, effective witness protection schemes minimise the danger of reprisals from mafia gangs.

The importance of convicting dangerous offenders does not justify abandoning a fundamental human right. Other common law countries - Zimbabwe comes to mind - will be all too keen to follow the UK's lead, and our standing on fair trials abroad will be seriously compromised. Ironically, this bill says that an order should be "consistent with the defendant receiving a fair trial". These are weasel words: President Bush said Guantánamo detainees should receive treatment "consistent with" the Geneva conventions, and they were water-boarded. The truth is that anony- mity for decisive witnesses is never consistent with a fair trial, and a law that legitimises it is a dangerous nonsense.